Earlier this year, Russian scientists announced that the "Test" experiments had found a range of different organisms that had been brought up from Earth and see

Earlier this year, Russian scientists announced that the "Test" experiments had found a range of different organisms that had been brought up from Earth and seemed to be surviving by clinging onto the ISS's hull. The above quote from the article suggests to me that there is a good chance that some of the bacteria remained on the hull long after the experiments performed by the Russians earlier that year.